The Radiant Heat Experiment – Did it Work?

One whole year ago, I was in the depths of destroying and rebuilding a sagging 1950s brick ranch house, which has since grown up to become our actual home. Looking back through Google Plus’s automatic archive of my phone pictures, I can see the “kitchen” was still open to the great outdoors on that date:


Despite the lack of windows and insulation, I was already looking ahead with nerdy engineering glee to building a home-brewed heating system for this place, and I told you about it in the article called The Radiant Heat Experiment.

In a nutshell, this involved running thousands of feet of PEX pipe under my existing wood floor via the crawlspace and circulating hot water through it with a pump and this high-end Rheem tankless water heater.

The plan was met with both enthusiasm (generally from fellow engineers) and scorn (more often from plumbers), and since then people have been sending in emails and comments to ask how it all went. Although I’ve already dropped a few hints that I’m very happy with the end result, this experiment came with some good lessons and pitfalls which are finally ready to share.  I have also had a chance to measure the performance of the system (and the house in general) through most of a Colorado winter, and the numbers surprised me just a bit. So let’s dig in.

How it All Went Together

Last time I presented you with a daunting list of parts. The list makes a lot more sense when you stick everything together. Here is a picture of the heart of my setup as it stands now, with everything screwed onto a plywood board:


The funny part is that all of the brains of the system are right there on the board. All the research and shopping boils down to just that 2×3 foot rectangle. The input is hot water from your water heater on the left, 120 volt electricity for the pump through the orange cord, and a pair of small wires you connect to your thermostat to indicate “ON”. Then the hot water flows out through the zones, delivers its heat to your house, flows back into the cold side of the manifold, and returns to the heater for another round. If DIY radiant heat were more common, this whole setup would come as a single product for $199 at Home Depot instead of the $600 or so you see here.

It took only about two hours to attach all this together, and then I confidently crawled down under the house with it to get to work on the rest of the installation. Little did I know that the real work was yet to begin.

Running the Heating Tubes

This system proved to be quite torturous to build, but it was because of plain old physical challenges rather than anything technical or mental. The problem is that to install radiant heat below the floor of a wood-framed house, you need to thread a huge length of stiff, fussy pipe through an unyielding grid of tightly spaced floor joists. I divided my house into six zones, each one about 250 square feet in area. For each of those zones, I had to do the same steps:

  • Meticulously review the underside of each joist bay and clear out any remaining scraps of duct work, old plumbing, spider webs, etc.
  • Grind off a few hundred flooring nail ends poking through from the original Oak floor above using a cordless grinder with a cutoff wheel
  • Drill a 7/8″ hole through the end of each joist
  • Pull through the whole required length of PEX pipe, fighting the stiff tangly coil the whole way
  • Staple it up to the underside of the floor, using aluminum reflector plates
  • Run the ends of the tube back to the manifold and connect them into the system
  • Cut and fit R-13 insulation batts underneath the whole area to force the heat upwards into the floor instead of down into the crawlspace.

The end result in each bay looks something like this:

Here's the end of one joist bay. Tubing, aluminum diffuser plates (optional), R-13 insulation batts underneath (essential)

Here’s the end of one joist bay. Tubing, aluminum diffuser plates (useful), R-13 insulation batts underneath (essential). The fluffy spray foam insulation on the right is part of my new crawlspace insulation – not strictly related to a radiant system but handy for keeping the resulting heat from leaking out through the rim boards of your house.

I found that each 250 square foot zone took about eight hours of work. But not just a casual eight hours that flies by like it does when installing kitchen cabinets while your radio plays happy bluegrass music in the background.  This is eight hours of proper torture, crawling in a 40-inch-high space with sharp rocky dirt beneath and obstacle-laden floor joists above. Even the slightest movement stirs up thick dust, so you have to wear a full-face respirator. That’s handy, since the grinder also throws down hot metal sparks towards your eyes and face. Gloves and kneepads are essential too. And ear protection. It’s dark down there, so you also have a bright LED headlamp strapped over top of all the other accessories on your head. But the ground-driven temperature of 60 degrees is far too warm for the work pants and long sleeves you need to wear to avoid skinning your arms and legs, so you also sweat a lot. In general, I could only withstand about 2 hours of this work at a time, so each zone was done over four days.

But if the paragraph above sounds horrible, you’re just thinking about it wrong. This is voluntary hardship at its best. The physical and mental benefits of crawling and sweating and fighting with stubborn tools and materials for so many hours are incomparable. Every possible move is constrained, so you must overcome the constraints with strategy and strain. The feeling of suiting up and descending into the crawlspace each morning while knowing I could earn much more money by outsourcing the activity and instead simply typing a bit more shit into this computer was enlightening. The feeling of emerging two hours later into the fresh air and bright sunlight, stripping off the dusty clothes and seeing the beauty of the world again was life itself.

Even with all that struggle and joy, I paused the effort* after finishing four of the six zones. Those cover the primary areas of the house and are more than enough to keep up with our heating needs for the rest this year. I’m finishing up the main floor carpentry and a second bathroom, and those last two zones will go in before next winter.

Real World Performance

The Living Experience

This was the unexpected surprise – how nice it is to have warm floors. Your feet get a pleasant reward with every step you take, or as you rest them on the wood floor under the dinner table. On top of that, anything you leave on the floor gets extra toasty: a pair of wet winter boots, a forgotten coat, or even the socks you threw off before hopping into bed – perfectly warm and dry when you pick them up the next morning. The bathroom floor also dries quickly after a shower.

Keeping up with the Cold

On a “normal” January day in my part of Colorado, daytime temperatures reach about 43F/7C, but the extremely bright sunshine makes it feel much warmer. The South-facing glass of the house sucks in about 10,000 watts of heat at high noon and it gets stored in the copious thermal mass of the various interior stone and brick walls. I blow it around with a ceiling fan to accelerate this process and the interior temperature reaches a peak of around 76F in the afternoon. Then the sun goes down, the stored heat is gradually released, and we make it through the night (low around 20F) with the house dropping to a comfortable 66. If you’re lucky, the sun rises into a clear sky the next morning and you repeat the cycle – with no heating required at all!

But weather adjacent to the Rocky Mountains is anything but consistent, and this winter we have also seen an all-time record low of -14F (-26C) as well as a daytime high of 77F (25C) just a few weeks later. This is why you still need a heating system with some juice.

With only four zones running at -26C, my house was a bit underpowered – the temperature would drop slowly unless we lit a fire (I also added a wonderful high-efficiency EPA woodstove to the house – another story). From a standstill, the system also takes about two hours to get the floors to their full operating temperature of 80F. However, the remaining two zones should provide the extra bit needed to keep up in worst case conditions.


To test this, I had to calculate the amount of natural gas I burned every hour, and compare it to the amount of heat actually being pumped into the house. I did this by cranking up the system on a cold day and taking “before” and “after” readings of the gas meter, and noting the flow rate and temperature drop** across the whole system:


Here are the things you need to look at to calculate system output and efficiency.


To make a long story short, the gas meter told me I used 40 cubic feet of natural gas over my 144 minute test period.  The gas bill tells me that each 100 cubic feet is 0.945 “therms ” (94,500 BTU) worth of heat. One therm costs 62.67 cents in my area. The net result is I was consuming 15,740 BTU per hour of gas, which is just under 10 cents worth per hour.

Next, I added up the (approximate) flows of those four flow meters and saw the system was pumping out 1.68 gallons per minute of water with a 16.5 degree F rise. You can calculate the energy delivered to the water with the “Universal Hydronic Formula” like this:

1.68 GPM x 16.5 degrees x 500 = 13,680 BTU per hour

Back in the design stage, this is roughly the heat loss I calculated my house would experience at a temperature of 20F, so the numbers seem realistic to me. Also, dividing the output by the input, we get a water heater efficiency of 88%, which is close to my unit’s stated efficiency of 94% (efficiency rises for lower input water temperatures, so I’m very happy it can perform this well with a 118F input).

On top of all this, I measured total electricity consumption (for the tankless heater and water pump combined) at only 55 watts, which is under $4 per heating month even if you run the system 24 hours a day. To add it all up, my home’s total gas cost this year breaks down roughly like this:

Gas company fixed monthly fee whether you use any or not: $12
Regular monthly gas use for showers, laundry, dishwasher, cooking, etc: $4
Heating for Oct 14 – Nov 12: $8
Heating for Nov 12-Dec 15: $55
Heating for Dec 15 – Jan 16: $58

And that’s probably the peak – here in February, the weather is already warming up and the system is off most of the time again.

So What’s the Catch?

When I started this experiment, I was optimistic that we could revolutionize home heating and make the forced air gas furnace obsolete. After all, the cost is lower, living comfort is greater, and you save a lot of interior space that would normally be consumed by ducts and chases – especially in multi-story homes. But until the industry advances a bit, there are a few flaws:

Building was Quite a Bitch

Installing this was near the limits of my skill and endurance, and I’m a not-all-that-old dude with lots of great tools who has been building things since I was a little kid.  However, it would be much easier if you installed it in an unfinished basement instead of a crawlspace. Also, recruiting as many friends as possible to thread the pipe will speed you up exponentially. Overall, I’d recommend it only for experienced handypeople.

Heat Output is Lower than Expected

I’m getting under 14,000 BTU per hour over the 1000 square feet I have installed so far. This works out to 14 BTU/hr per square foot. This place is pretty well insulated, so I should be fine. But an older and draftier house would lose more heat. The problem lies in the slow transmission of heat through the 1.5 inches of my subfloor and the oak floor above it. To increase that, I’d need to raise the water temperature further (it is already at 140F) or add some extra radiators.

On the positive side, you can get really creative with radiant heat, embedding the tubes into tile walls, or making heated towel racks in your bathroom that tie into the system. Each extra heated feature will deliver more BTUs. Also, installing under a tile floor instead of wood floor will increase heat transmission.

Not all Tankless Water Heaters will Work

In reader feedback, I heard stories of tankless heaters dying early or cycling constantly. Cycling is not a problem with the unit I used – it runs at variable speed so the system quickly reaches a nice loafing steady state where the pump is going slowly and the heater is barely murmuring to match the required flow and temperature rise. Time will tell how long it lasts, but I’m betting it will prove to be far more cost effective than a $3000 boiler.

The Open Loop System Has Drawbacks (as well as advantages)

I am using a single tankless heater for both home heating and domestic hot water – this is called an “open loop” configuration. It would be easy to add a second basic heater for $600 for the domestic water. This would separate the water systems, and I may do that someday.

The main drawback of combining them that you need to keep the water heater set very high (140F) to get enough heat output to the floors. This means somewhat fussy water temperature balancing in the shower, whereas with a dedicated tankless heater you just type 110F into its remote control unit, crank the hot water handle, and enjoy a computer-regulated perfect shower every time.

A second issue is that the hot water can sometimes smell like new plastic pipes. This effect faded to zero after about three months, but it is worth noting, especially if you are installing your system in a house with people likely to complain about this. All components I used are after all specifically designed for potable water.

On the positive side, I found that if you run hot water when the pump is off, water is drawn through the system through natural pressure differences. This means that in the summer, my floors will actually be cooled down by the cold water supply as it sucks unwanted heat from the house. So the floors will pre-heat the water before it hits the water heater. Double energy savings and free air conditioning.

Because the water supply is constantly refreshed and/or heated to 140 degrees Fahrenheit, bacteria growth and stale water in the pipes is not an issue at all.


It has been a worthwhile experience. Loads of learning, plenty of hard labor, a luxurious end project, and an $8,000 savings over having a new forced air furnace and duct system installed into this house. Although DIY radiant heat is not for everyone, I can declare this particular experiment a success.


* To finish up next fall, I will also swap out the manual adjustment dials (white knobs in the picture) for electrically controlled actuators, and use a multi-zone WiFi thermostat to control the whole house. This thermostat is being developed by an MMM reader who has started his own company to produce it – more details on that in a future story.

** The temperature drop is configurable with a little knob inside the computer-controlled circulator pump by Taco. I set my own pump to maintain a differential of 20 degrees F, which is typical for a system like this. Then if the pump starts seeing a drop of more than 20 degrees, the pump runs faster to compensate. If it is less, it means your house is already warm so the pump runs slower.


  • Robin February 6, 2015, 8:06 am

    This is awesomely detailed as usual. I’m sending this to my husband because he has been dying to DIY this in our own home, and I keep discouraging it. I guess I should suck it up and let him do it…would I ever really complain about toasty feet on hardwoods? Probably not.

    • josh February 6, 2015, 8:34 am

      Good to see success. A similar alternative is to pair a hydronic air handler with a tankless water heater. Its the same principle except the in-floor pex is replaced with a furnace that has a water-to-air heat exchanger (instead of a burner). This might be a good alternative if you need to keep your AC coil and duct system, have a basement, have thick wood/carpet or otherwise insulating floors, or need a higher output for a cold climate (say 40,000-80,000 btu/hr). You can also get the return temperature very low, which will increase the efficiency of your condensing water heater (if you have one). We completed a study of these types of systems a few years back. They work well, but as noted in the article, you need to pick the right water heater. Cycling at low loads can destroy many different parts of poorly designed units.

    • Free Money Minute February 6, 2015, 12:06 pm

      I agree. I am just wondering if this will work under the brick floor I have in my living room on the far end of the house. It gets pretty cold in that area during the long Michigan winters. Any thoughts on whether it will heat the brick up and thus create a warmer living room on that end?

      • isaac Aeppli February 6, 2015, 12:35 pm

        I am wondering if you have two floors, did you run this system on the second floor or is it just in the basement area. and also if it isnt ran on the second floor, how well did it heat the second floor?

      • former player February 7, 2015, 2:33 am

        My understanding is that underfloor heating works best on solid floors such as stone and brick, so yes. Installation of underfloor heating is a doddle in a new build, and easier than the MMM method in a refurb if you take the floors up, so I’d probably suggest taking the floor up, installing the heating and then relaying the brick floor.

        I have a rather simpler to install (but more expensive to run) electric underfloor heating in my garage conversion – stone flagged floors – and it is wonderful.

        • Pete Peterson November 2, 2015, 7:33 pm

          I really enjoyed the information since I am where the MMM project was a year ago. 6- 250ft 1/2 in PEX loops under a 1200 sf 1938 home that has extensive upgrades (r-19 walls & r-23 ceiling + high efficiency windows and doors) here in the hills of north central Arkansas. Also 1- 300ft loop under a slab installation as part of the over all project. Test run this past week for leaks & water circulation, Will be happy to see the results when the winter gets in full gear. I can personally attest to the PAIN of crawl space PEX installation. It nearly got the best of this 80 yr old dog,

          • Darby December 28, 2016, 9:54 pm

            Wondering how this worked for you Pete? My building configuration is very similar to yours.

    • Dan December 16, 2015, 8:38 am

      Awesome. Thanks for the great write up. I too am an engineer and I designed and built a similar do it yourself, pex radiant system in a 2000 sq ft, passive solar, super insulated, ranch style, concrete slab house we just built in upstate NY. My system is similar except I have adjustable mixing valves to knock down the water temps for potable use and for the floor heat delivery. I have a high efficiency condensing propane tank hot water heater. My concrete slab is a huge thermal mass. The heat has to be on for many hours straight to make the slab warm and move the air temp even one degree (supply temp 100 deg F, return temp 80 deg F).

      Some issues with my system:
      1. I have only one “zone” for the whole house. In the south facing rooms with lots of glass, the temps increase during sunny days but the North rooms stay cool. I may be adding another thermostat and a valve to zone the North side of the house separately.
      2. I have a vibrating noise made by a check valve, I need to work on that annoyance. (maybe trapped air or turbulence in the pipe).
      3. When I used a standard issue 40k Btu propane water heater, it could not keep up with heat and showers at the same time. I upgraded to a 100k Btu high efficiency condensing tank. It was affordable only because I lucked out and found a dent and ding unit on Craigslist that is perfect for my application.
      4. Because the passive solar keeps the house warm on sunny days, the system can go a day or two without the thermostat calling for heat. The floors go cold and my wife protests: ‘you promised me warm floors’. I tell her this is a good sign that we are getting “free” heat from the sun, but cold toes outrank energy savings in her world. (I have an option to put a thermostat temp sensor in the slab and use that to trigger the heat but I’m afraid that will overheat the air temp on sunny days.)

  • Mrs PoP February 6, 2015, 8:11 am

    ha! That picture of your kitchen is about what ours is looking like these days. Not having a ceiling makes for chilly mornings in the winter, but I expect it will be well worth it when we’re finished! =)

  • Mr. Frugalwoods February 6, 2015, 8:11 am

    Thanks for the report back, I was waiting to hear how it turned out!

    At least here in New England, radiant floor heating is falling out of style. Mostly because of installation costs, but also because of the lack of efficiency when compared to the new generation of air source heat pumps.

    I’m curious, did you consider an air source heat pump for your situation? I imagine you are in their temperature extreme, but they are installing them up in central Maine so I would have to guess your climate would also be appropriate. The nice thing is that they are super cheap for initial costs, and since they run on electricity you can offset their energy appetite with solar.

    Natural gas prices are also much higher here than where you are, so that plays a major part. Especially if you go completely gas free and don’t need to pay that monthly minimum all summer long :-)

    We’re gas hot water baseboard in our current house, but I think in our next place we’re going to seriously consider the new heat pumps.

    Anyway, thanks for the update. Always great to see someone else who nerds out as much over hvac as I do!

    • tcmJOE February 6, 2015, 3:48 pm

      I was wondering the same–I know heat-pumps can get some fantastic efficency, so I was curious if they were worth the cost.

    • jpmitchell February 7, 2015, 3:52 pm

      Ductless Multi-Zone air sourced heat pumps! Heat pumps use heat transport (like your fridge but in reverse) instead of directly heating by burning fuel and as a result they can achieve efficiencies significantly higher than 100% (i.e. for every unit of electrical energy in, you get more than one unit of heat energy out). In the past they didn’t work well in cold climates because they become less efficient as the temperature gradient between inside and outside grows (like really cold winters), but modern heat pumps have gotten much better and are now good down to about 5F.

      Sure, you won’t get that luxurious warm floor feeling, but they are without a doubt the most efficient heating/cooling solution out there, and coupled with their relatively low upfront cost they just make sense.

      If you’re technically inclined, check out David McKay’s “Sustainable Energy Without the Hot Air.” He discusses heat pumps (and a lot of other really awesome stuff) in great detail.


      • theFIREstarter February 9, 2015, 12:18 am

        +1 for “Without Hot Air”!!! Great book and it’s free online as well.

        Coincidentally we all just had a pretty good discussion about Air Source Heat Pumps over at my blog here: http://thefirestarter.co.uk/should-we-buy-an-air-source-heat-pump-or-new-boiler-plus-the-gdhif/

        Bear in mind it’s a UK based discussion, but a few key point that came out of it were:
        1. If you have an old working boiler you are better off keeping it running for as long as possible and making the rest of your house bleed less warmth into the outside. This may not hold for bigger American houses but our little brick built houses in Europe are probably easier and cheaper to insulate (Our house is ~1000 square foot, it seems pretty massive to me). Likewise if you are connected to gas mains they say they are probably not worth it right now as the £££ savings do not cover the install cost. (purely from a financial point of view). They are highly recommended for off gas grid locations where they would replace oil boilers, biomass boilers, etc…
        2. ASHPs are pretty expensive over here! I would be interested to hear any comparison prices for the pump plus full install from the US? (You could/should obviously install it yourself to save £££/$$$ though)
        3. Saying that, the government are trying to encourage people to get the ASHP by offering payments back to them on the heat generated. But if you buy an ASHP not through this scheme they seem to be much cheaper, so maybe that is a bit of a swindle! Wonder if you have anything similar over there?

        I’d very much like to hear more opinions on the ASHP, from people who have installed them. Operating temperature/efficiency/cost and things like that.


      • tcmJOE February 13, 2015, 12:54 pm

        “Without Hot Air” is my go-to guide for all environmental and energy discussions.

      • Emily zefrencharchitecte August 14, 2015, 2:35 pm

        There exists such a thing as a heat pump for radiant flooring, it is an air/water heat pump (a part of it is outside and pumps in calories from the outside air, the other part puts that heat into your radiant pipes)
        It works best for low-temperature radiating (the max temp of output for efficiency is close to body temperature; whatever that is in farenheits) so exactly perfect for floors/walls etc, less so for wall-mounted hot-water radiators or convectors that are typically or even hot water for human use because the temp needs to be higher to kill off / not let germs proliferate.
        Heat pumps also contain greenhouse-causing gasses, and are more sophisticated beasts than boilers or gas-burners…
        They DO convert 3-4 times (300% 400% depending on outdoor temp mainly) the electric energy they use into calories (BTUs or Joules or other units) – more efficient than even condensation boilers (120% tops)

    • David August 18, 2016, 7:37 pm

      In my part of Maine I don’t know of many heat pumps because they don’t do much in subzero temperatures. Radiant heat is what I see most in new construction. I know of a few homes in Augusta with heat pumps and they all have an additional heat source for cold days and most nights. Secondary heat is usually from electric baseboard or Rinai direct vent gas heaters.

      • Sidney August 16, 2018, 2:45 pm

        I think air source heat pumps are becoming more common now (in Maine). There are models which work at -15F (-26C). Probably still need a backup for colder days.

  • Steve Adcock February 6, 2015, 8:11 am

    This is an awesome experiment, and one that you may have motivated me to try – though I am admittedly less skilled with my hands than you are. “Building it was a bitch” will be, by far, my biggest problem! Saving $8k? Not bad.

    It all counts.

  • Chris February 6, 2015, 8:17 am

    Well that is flipping awesome. I know we love our wood floor for their cooling effect. We only use the A/C at night in the summer. But during the winter, I’m wearing socks and slippers most days since it feels so much colder! Very neat project.

  • Max Misfit February 6, 2015, 8:19 am

    Well done! I am a Mechanical Engineer and always find it fascinating to see people tackle DIY projects that involve basic mechanical components. I am actually about to take on the challenge of changing out my vehicle’s AC compressor and putting in new Engine Mounts. I work on large industrial mega projects as a consultant, so it has been a while since i have been under a hood. I will try and document the experience and share on our site. Keep up the good work!

    • Mattattack November 25, 2015, 10:38 am

      I changed the compressor (along with dryer and expansion valve) on my ’94 corolla. Its really not that hard. Its just a pain because they cram everything into such a tight space. When re-installing the expansion valve assembly, I accidentally crimped the condensation drain inward so after a 45 min drive to the hill country I had a puddle of water in my front passenger seat. That was a pain.

  • Eric February 6, 2015, 8:20 am

    Awesome write up. Would you consider using a section of cast iron baseboard or slantfin to finish up the system? Seems like it would be a lot easier to run.

  • PatrickGSR94 February 6, 2015, 8:26 am

    I probably missed it from that pic of the unfinished kitchen, but why is there no sheathing on the walls? Looks like the back side of the brick veneer is visible everywhere, right through the studs.

    • Mr. Money Mustache February 7, 2015, 2:47 pm

      In this rebuild, the shear strength comes from the 4×6 steel beams and columns that I ran along that wall. Everything is welded together and the wood is bolted into the steel. Bring on the hurricanes!

      • Corey September 20, 2015, 4:48 pm

        Did you do an interior air barrier? What kind of material?

  • Dan Zehner February 6, 2015, 8:38 am

    We don’t have a basement or crawlspace in our house, but this looks really intriguing for heating my shop… :-) How would you go about laying out the piping under an existing concrete slab? Just build another layer of flooring on top?

    • Mr. Money Mustache February 6, 2015, 9:00 am

      Yeah – that’s actually what I will be doing for Zone #6 here in my house. My office is in a part of the house with a concrete floor (it was built with the option of being a garage). I was going to stick some tubes down on top and pour concrete over that. This will also help with leveling the floor, as it is currently sloped down to an old floor drain.

      This will waste some heat as it will seep into the soil below as well as upward into the living space. But I can insulate the soil from the outside to reduce this effect, and eventually I’ll be pumping in free solar heat from a water-based collector. The idea is to see if the whole chunk of concrete and soil can act like a 100,000 pound thermal counterweight.

      • Tuxedo February 6, 2015, 10:27 am

        Great article! I was curious to see how this would work out.

        I wonder if it the cost of heat loss would justify installing 2″ rigid insulation on the existing slab before installing the PEX and new concrete? Around here Durofoam goes for around $0.80 / sqft.

        • Mr. Money Mustache February 6, 2015, 12:03 pm

          That would definitely pay off, but then I would have a 2″+ step up into that section of the house. I’d rather lose a bit of heat.

          • Mike February 6, 2015, 1:49 pm

            Last year, when I was looking at how we were going to remodel a home we purchased, I came to this same point. The home is on a 4 inch slab, and had been retrofitted with pex floor heat, above the concrete with 1x framing. We resolved to remove all interior walls, lay fresh pex, and repour a slab.

            We ended up pouring a 3 inch slab over the existing, and I discovered a new product, SlabShield, which as I recall was approximately 1/4 inch thick, and made up of thin insulation laminated with foil. I layed this down, rolled out 6″x6″ mesh (to secure my pex to with zip-ties) and then layed down my pex loops. Fresh concrete was poured on top.

            The SlabShield then acts to radiate the heat up, and prevent heat from dumping into the old, 4 inch slabs. I also used triple layer Pex-Aluminum-Pex, which holds its shape when formed, and made install quite enjoyable.


            Hit me up for any questions or pics if desired, and thanks for the update on your system, I’ve been interested.

          • DK February 7, 2015, 8:30 am

            Why go a full 2”, when I did my basement I put down 3/4 hi density foam board as the first layer (I didn’t want a big step up either). A little bit goes a long way with that. I even had some scrap 1/4 that I stood on that, then onto the slab, then back onto it just to get an idea if it would be worth it. huge difference.

          • Emily zefrencharchitecte August 14, 2015, 2:52 pm

            Ok so then as mentionned twice a thin reflective barrier, or you could jack-hammer through the slab (post pictures !) and start fresh with good insulation?
            Vertically cutting off the earth beneath the slab by digging trenches outside of this zone + putting in rot-treated insulation + finding a way to elegantly protect the insulation from the sun (UVs) and sight seems just as much work…
            Oh and do you still have zones that are on hardwood floors ? because as badass working in a coffin-like environment 2 hours per day is, I think you deserve to have a better experience next time. Try pulling up (for reuse of course) the original flooring, put down a subfloor on the beams and joists (or keep an existing one) – perhaps the handy product Mothe fussbucket mentionned- lay down the pipes so much more easily, then put back “reclaimed” flooring (on the grooved product or on simple spacers you screw into, at regular intervals if you choose a normal subfloor) you can fill in the spaces with sand or fine gravel, for extra thermal mass (but longer time to reach max temperature of course as mass will “absorb”)…

          • Al Waltrip February 23, 2016, 5:35 pm

            Great article. So I have a 50 gal electric water heater, do you think this would suffice for a 1200 sqft house and 2 people showering a day? Just curious.

            Thanks in advance.

      • Mother Fussbudget February 6, 2015, 2:00 pm

        Why wouldn’t you install a thermal barrier (foil) directly on the slab, then plywood subfloor with pre-routed PEX grooves, and a finish floor over that? (hardwood, tile, pergo… whatever) That’s how the This Old House guys do it in New England

  • Anna February 6, 2015, 8:45 am

    I was at a startup competition in SLC recently and heard this professor from the U pitching for start up capital to produce this product that is a replacement for forced air, but cheaper than radiant heating. It was so fascinating: https://www.youtube.com/watch?v=mJKKsH-YUh8

  • Bruce Mc February 6, 2015, 8:47 am

    I’m a new reader of MMM. This project looks amazing. I noticed your comment about how nice it is to have warm floors. My floors are just the opposite – they get cold in winter. A few years ago I started wearing indoor footwear that have a thick foam sole, which makes a good insulation barrier between the bottom of my feet and the floor. That makes the place much more pleasant, and actually cuts down my heating bill.

  • Paul Taylor February 6, 2015, 8:47 am

    I installed radiant in-floor and under-floor heating in a home I built about 10 years ago. Here is what I found out.
    The project was completed by myself, my wife and one employee. The lower level of the house, with a concrete floor poured after the PEX was installed, topped with ceramic tile, was easy, fast and worked like a charm. The upper level, engineered joists and sub-floor, topped with hardwood,was much more time consuming (maybe 2X-3X) and worked nowhere near as well. The lower lever would heat up n a very short time and retain that heat for quite a while. The upper floor (separate zone) would take longer to warm and required more frequent cycling of the pumps to maintain a given temp.Were I to do this again I would use the radiant in the concrete floor and min-splits on the upper level.
    My best advice: sun orientation and roof overhang for shading are the most important consideration when deciding on the heating and cooling requirements for a structure. For those of you interested in a serious DIY take, find a copy of Your Engineered House by Rex Roberts (long out of print.)

  • Jeff February 6, 2015, 8:48 am

    Pumped water heating is the common system over here in Britain.
    Most of the system is filled with corrosion inhibitor and the hot water is separated, as it’s usual to have an indirect coil passing through the hot water tank.
    I think you need to consider the corrosion risk with your system.

    Anyhow, well done with the under floor heating part. I would like to change from my radiators to that type of system.

  • KMB February 6, 2015, 8:49 am

    MMM, couple of concerns
    “This means that in the summer, my floors will actually be cooled down by the cold water supply as it sucks unwanted heat from the house”

    I know you guys are pretty dry out West, but if someone were to do this somewhere with high summer wet bulbs, should we be concerned with condensation on the floors, or even worse, condensation IN wood floors?

    Also, how do you think this system will affect resale value? This design is unique enough that most buyers will not have a thorough understanding of the system. Contractors likely will be confused by the novelty as well. Any thoughts on this?

    Finally, how often is water purged from this system? Only when you run the hot water from the tap? Do you have a strainer? Do you have any concerns about water blockage at all?

    • Emily zefrencharchitecte August 14, 2015, 3:07 pm

      “if someone were to do this somewhere with high summer wet bulbs, should we be concerned with condensation on the floors, or even worse, condensation IN wood floors? ”

      yes. the condensation would occur on the surface of the piping, so in MMM”s case, inside his insulation. not good at all as the water would probably be trapped by the kraft paper and little air cicrculation (rock or glass wool is insulating because it keeps air from circulating….) and would just soak up like a sponge and rot the ajoining wood.
      To avoid this in a humid room, one would put down continuus vapor barrier (floor, walls, ceilings) just beneath the “seen materials”.
      If it is the ambient air that is humid (ie the climate), you can’t keep this from happening. If the water circulating in the pipes is at ground temperature it will cause condensation. If you poor out a glass of water from your tap and it dripps with condesation, your climate is too humid to raidant-cool in a putrescible (rotting) material.
      If you do it over a slab and under tile/stone/brick, your floor will be slick and too cool to walk barefoot, in my opinion. Better use radiant ceilings for cooling, radiant flooring for heating.

  • TheMilkman February 6, 2015, 8:50 am

    Did you look into Geothermal systems when considering a heat source for your water? I’ve been looking into them for a new house that I’m building and it seems like an interesting idea to use a desuperheater on the geothermal system to heat the water.

    • Mr. Money Mustache February 6, 2015, 8:55 am

      Water-based geothermal heat is a great idea for situations where you need a lot of heat (for example, a larger house in a colder climate). Also, anywhere you are stuck with a more expensive heat source instead of my area’s cheap natural gas.

      Mr. Frugal Toque had a system like that installed into his rural mansion outside of Ottawa, Canada where some people have annual heating bills of $4000 or more – I’ve been meaning to ask him if he’d write a post on the article.

      • Yossarian February 6, 2015, 9:41 am

        Can confirm, am Ottawan, it’s a lot of cash. In winter months we use 1000-1400kWh of electricity which is roughly $250 at the high end, but that’s for a small apartment.

        By comparison we use ~200kWh in summer (hot water included) so it averages out nicely.

        If housing prices weren’t so inflated I might own a house so I could see some insulation ROI.

        • Cheap Mom February 6, 2015, 1:24 pm

          (another Ottawa reader) $250 for an apartment? Man, I’m glad we have natural gas! Our bill for our house in January was $104 (we do wear sweaters and slippers at home thought).

          Our furnance might need replacing in the next 5 years and any alternatives like this would be interesting to look into. I think with our cold climate though the standard gas forced air option might end up being the most economical. My parents have radiant heat in their bathrooms and having toasty feet is definitely nice.

          • Yossarian February 7, 2015, 6:07 am

            Averages out nicely to just under $100/month year round, and that includes our hot water, but out of context the winter months do seem absurd (and we do keep the place at 15 degrees max). No matter the math I do, living here is cheaper than buying a house/condo in the current market.

            The place is terribly insulated (1890’s fourplex) but I can’t do anything about it, which is the only perk of ownership IMO.

            • Kenoryn February 9, 2015, 10:52 am

              I lived for one year in an 1890’s apartment with baseboard heating. That was expensive. Our electricity bill was about $450/month in the winter – to keep most of the apartment at 15, with one room heated to 20 degrees, only in the evenings. In Peterborough, maybe a degree or two warmer than Ottawa. ;)

            • Emily zefrencharchitecte August 14, 2015, 3:18 pm

              You can seal off your windows in winter with glorified Saran Wrap.
              make sure your front/common door is snug all around, you CAN offer the landlord to accept a week of workers in your appartment and give him/her what you save on your electric bill (up to 20 dollars a month perhaps and you keep the rest if there is some). Make them pay for new paint and buy the “reflective” paint (not sure if it works, i am a natural sceptic). get rugs.
              These are tricks I learned as a student living downtown in old buildings. (Montreal and Quebec)
              Oh and I am 6 months late, but this is the time to have insulation put in.

      • Mr. Frugal Toque February 8, 2015, 9:25 am

        Yeah, I really should write an article about that. I took so many cool pictures and videos, it seems like it would write itself.
        I don’t have the numbers easily accessible right now, but the system installed was something like $32k. Another $1k was spent on electrical wiring with some ridiculous 60A and 100A cable and something like $3-4k was spent repairing the landscape. At the time, the provincial and federal gov’ts were offering about $8k in rebates.
        My estimated savings were just over $2k per year (I should really come up with more exact figures) but I also benefited from the removal of an incompetently installed oil furnace (leaking oil pipes, an exhaust vent placed directly below my kitchen fan, etc. etc.)
        Was it a good investment purely from a financial perspective?
        Possibly, but maybe not.
        Do I breathe cleaner air and walk around with an air of moral superiority?
        Damn right.

        • theFIREstarter February 9, 2015, 12:37 am

          Presuming you are talking about a Ground Source Heat Pump here?
          Would love to see your write up on that Mr Frugal Toque!

      • Elaine November 27, 2015, 5:31 pm

        We installed a geothermal system in 2009. It was coming to the time to get a new furnace (3 repairs over a few months), and the governments were offering huge rebates from a variety of programs. We live outside of Haliburton, Ont., which is about 4 hours west of Ottawa, and directly south of Algonquin Park, so winters can be very cold.

        We went on a “hot date”, as we later called it, one evening to a presentation by a local builder, and signed up the next week. We are on about 6 acres of land. A big trench, 6′ deep and 6′ wide in a loop totalling 600 feet was dug on our property in a field that we leave to nature, and 2400 feet of pipe was installed. The pipe is a closed loop containing water and some sort of antifreeze. The ground at that level is a constant 10-12 C year round. The inside unit circulates the air and takes out the warmth in the winter, and outs warmth back in the summer. The air is less hot than that produced by an oil-fired furnace, and much more comfortable. The system has been trouble-free, and the only maintenance is a monthly vacuuming of the air filter. The inside unit is made by NextEnergy.

        Total cost was about $30,ooo, but we got 1/3 back in grants, and we would have been in for a new furnace anyway. The local installers asked me to prepare some figures for them this summer. It looks like, even with the additional electricity costs, that we’re saving something in the neighbourhood of $1,500 per year in energy costs. It’s also great to know that we’re not relying on fossil fuels for our heat.

        • Elaine November 27, 2015, 5:36 pm

          It should have read that the inside unit circulates the water in the pipe system.

          • Elaine November 27, 2015, 6:37 pm

            The water pipes can also go 150 feel straight down into the ground if you don’t have enough land to dig the trench, or can be installed into a lake (of which our county has 600) or river.

            6′ x 6′ x 600′ is an awful lot of earth, but it’s amazing how quickly things grow up again once it is all filled in and smoothed out.

  • Wiggles February 6, 2015, 8:54 am

    What a cool idea! How did you decide how many lines to run between each joist? It looks like you ran two rows on lines between each. Did you contemplate running a third row? Is there any efficiency to be gained running more rows?

    • Mr. Money Mustache February 6, 2015, 10:23 am

      I did two because that’s what it showed in the books and I figured it would help maximize heat transfer. I’m not sure if three would be any better and it would be much harder to do that in a 16″ joist bay (more fiddling with pipes, tangling, etc.)

  • Patrick February 6, 2015, 8:59 am

    I knew something was wrong when you said 1000 cubic Ft. Of gas was 62 cents because a 1000 cubic feet sells from energy companies to utility companies for 3$-4$ right now.

    It looks like you added an extra zero. 100 cubic feet of gas equals 1 therm equals 100,000 BTUs.

    1000 cubic feet of gas equals 1,000,000 BTUs.

    Your efficiency math appears to remain correct because the way I read your gas meter you only used 40 cubic feet and not 400.

    • Mr. Money Mustache February 17, 2015, 11:57 am

      Good catch, thanks Patrick. I had misread the gas meter and that last dial says “1 thousand per rev”. I was thinking it meant one thousand per number. Luckily my mistakes canceled out, but I have fixed all of it in the article now.

      1 CCF (100 cubic feet) is around one therm (100,000 BTU) and usually costs about a dollar retail in more normal market conditions outside of the current cheapness.

  • Joe February 6, 2015, 8:59 am

    How would you deal with water leaks? Detecting them before they become an issue and fixing them?

    • Mr. Money Mustache February 6, 2015, 10:22 am

      I ran each zone for a day before tucking in the insulation and verified that none of the joints leaked. There are very few joints as everything is done with continuous lengths of pipe.

      I’ve never had a PEX joint go bad after this stage, but if one did start dripping I would eventually see wet dirt in the crawlspace and know it needed repair. Just as with any other plumbing joint in a house. Note that the entirety of house and building construction depends on pipes being EXTREMELY reliable, so they are designed that way. If they weren’t, you would end up with very frequent floods in multi-story buildings destroying everything below them. It happens in the event of flawed workmanship, but it is rare.

  • Thegoblinchief February 6, 2015, 8:59 am

    Very cool. Probably not something I will do in my current house, though if I did do it, I have a mostly unfinished basement (and the finished part needs to have the crappy drop ceiling replaced anyways), so the installation would be significantly easier.

    I think warm floors would make my wife very happy, I just don’t think we will be here long enough to justify it. Too many other projects to get to first.

    Thanks for sharing the update.

  • TheFamilyFinder February 6, 2015, 9:06 am

    Great article. I am also wondering about the moisture build up from condensation. It can be very humid in Missouri. We tried last summer not to run our AC unless unbearably hot (95+)in attempt to be a little Badass. We found that condensation was a BIG problem and ended up with a mold in the basement. LOTS OF IT. Next summer we are thinking of making our AC decisions based on the humidity level instead of the temp to avoid the mold again. I will be following as your experiment continues!

    • Katie February 6, 2015, 12:43 pm

      Do you have a good dehumidifier? I have one in my basement, it ended up running continuously for about 3 months but it did keep the humidity at a more reasonable level. Also smelled better.

    • FrauSchnurrbart February 7, 2015, 2:22 am

      Here in Germany, we also have high humidity (though not the heat). We air out the apartment daily.

      We keep digital hygrometers in all roomst hat light up a frowny face (very cute and inutive) when humidity gets above 65%.

      Airing out in summer is another story, that’s better done only at night….

  • Lakemom February 6, 2015, 9:10 am

    Could a similar tankless heater with circulation pump be configured to replace an ancient (1961) boiler system? With no need to reroute all the current baseboard style radiators? I’ve been thinking ‘casually’ about this since you early posts on your engineered system. Every couple of years when we have our boiler services the guys tell us that it would be expensive to replace and even though the current one is only 60% (ish) efficient a new one would take decades to ‘pay for itself’ via savings on natural gas costs.

    • Mr. Money Mustache February 6, 2015, 10:10 am

      I’d definitely give it a whirl if I were in the same situation. The only risk is that the tankless unit might not last as long as in a normal service life.. and that it might not run at peak efficiency. However, with a high-end Rheem commercial unit like I used in my system I am optimistic in both areas.

      According to other readers who have sent in helpful feedback after the first article, boilers require regular servicing which can cost more than running a tankless heater, even if you had to replace it every few years.

      • lakemom February 6, 2015, 10:18 am

        Thank you! I’m going to file this away in my research file for when we either “need” it or have the time to tackle it. Does anyone suppose we could get an actual plumber to do this for us? While we’ve done just about every task you can come up with, I’m not sure the dh would be up to wanting to tackle this one….living as we do in the upper Midwest with very cold winters.

        • Kenoryn February 9, 2015, 12:06 pm

          Another thing you might want to look into is condensing water heaters. They are hard to find and unfortunately most residential installers don’t seem to know much about them, as they’re generally used for commercial applications and for in-floor heating, but I bet it could be worth it. They’re more efficient than tankless units in some cases, but also a little more expensive.

      • Dave February 6, 2015, 6:04 pm

        My personal solution to the same problem is to use a atmospheric gas fired boiler with a four way mixing valve. I have a computer that controls the four way valve to vary the baseboard water temp based on outdoor temp. Atmospheric boilers tend to be very low maintenance and lack the problems that condensing boilers and tankless hot water heaters have with scale buildup in the heat exchangers.

    • Texas Jim February 6, 2015, 12:07 pm

      We looked carefully at a tankless water heater when we replaced our aging system. What made it prohibitive for us was that we don’t have gas service and upgrading our electrical to handle the tankless system pushed it beyond our practical limits.

      • Jeremy February 2, 2016, 11:19 pm

        I don’t have gas service but installed a Stiebel Eltron Tempra Plus 36 electric tankless unit this past November 2015. It has provided enough hot water for our 2 bath household but am looking to install a closed system using a dedicated electric tankless water heater for radiant floor heating.

        Does anyone have any experience or advice using an electric tankless water heater in a closed system? I’d be interested to know GPM needed and volts or amps. If going with Tempra, is 12 or 15 Model sufficient? ( http://www.stiebel-eltron-usa.com/sites/default/files/pdf/sizing-guide-tempra.pdf) Thanks for all of your help!!

        • Jeremy February 22, 2016, 6:25 pm

          Totally different Jeremy but I’m in the exact same situation, though our Tempra 36 was installed in September. Curious to have your (and my) questions answered regarding feasibility of Tempra 12 or 15.

  • Mr. 1500 February 6, 2015, 9:28 am

    Awesome! I’ll be following in your footsteps (kneesteps?) shortly. We currently have a big nasty furnace and another big nasty hot water heater in the crawlspace. I’ve been dreading the day when either of those decides to take early retirement. This solution will ensure that I never have to jimmy a furnace in down there. The other nice thing is that I can set it up now and just flip the switch when the furnace or hot water heater dies.

  • David February 6, 2015, 9:42 am

    This is very cool. I was telling a contractor about this and they said that it can dry out the wood floor. Is that a problem, or just a rumor that circulates that community?

    • Katie February 6, 2015, 12:45 pm

      Wouldn’t a good humidifier counter that?

      • Mr. Money Mustache February 7, 2015, 8:12 am

        I agree – it has indeed dried out my floors and I can now see a few small gaps. In higher humidity, it will swell back up and the gaps will close.

        Because this is a 60-year-old floor, it also squeaks a bit when the system is on, even though I had meticulously screwed down all the squeaky boards from below before installing the system.

        These are the drawbacks of having an old, wood-framed house. If building from scratch I would make a house entirely from concrete and have the heating embedded in the nicely insulated slab floor.

        But all of this is of course just a case of Tiny Details Exaggeration Syndrome – to even own a house at all, let alone one with automatic awesome heating of ANY type, is a pretty fine addition to life.

  • Bret February 6, 2015, 9:58 am

    My dad installed radiant heat in the house he built in 1998 and it’s worked very well over the years. The pipes run through the concrete in the basement and in a self leveling concrete underlayment upstairs. The same boiler that heats the house also heats a water tank for house use, and it’s very efficient. The only problem with it is if you (or a teenager) insulate the floor with piles of clothes, then the room gets cold.

  • Gordo February 6, 2015, 10:01 am

    Sounds like a fun project. I know you already have a woodstove as well. I think it may be much easier for some people to just heat with free wood – but the ongoing effort can be considerable. I documented my experience heating with only free wood here:
    Same stove is still going strong to this day (it went down to 6F last night) without any problems or major repairs needed. I got a year’s supply of free wood over the summer from a SINGLE tree that a homeowner had removed.

  • andron February 6, 2015, 10:01 am

    Here in Canada for open systems like yours we are required to have the entire system flushed at least every 24 hrs to prevent bacterial growth/ plastics leaching. While the system is ingenious it is definitely unsafe and inefficient (combustion appliances are very inefficient when run at minimum fire when they aren’t designed to do so)

    To all others who want to copy: please check local codes (they are there for your safety!)

    • Mr. Money Mustache February 6, 2015, 10:19 am

      Andron, I’m skeptical about your claim about efficiency, since I measured 88% even with my system only 2/3 built! (And it should go higher as I move to a higher firing rate with the other two zones).

      As for bactarial risk: do you have any sources to back up your claims? I read quite about about Legionellia before building this, and it really seemed inapplicable.

      After all, in a standard tank-style water heater, the water sits at 120F-140F and may do so for weeks when you go on vacation. Here, the water is either circulating at 140F, or sitting at ~60F when the system is off. And the whole system gets flushed out every time someone uses hot water for shower, laundry, dishes, etc.

      Regardless of this mini-debate, the open/closed loop system is a minor decision in the main context of radiant heating. If you are concerned, just add a second water heater for your domestic water, to separate the heating loop (as already mentioned in the article).

      • Shane February 6, 2015, 11:24 am

        I think an easier solution is buying a heat source (boiler or tankless heater) that has two loops so you only have 1 source of combustion but have a closed loop system. Another feasible solution is adding an indirect water heater to separate your domestic water from your heating water.

      • andron February 6, 2015, 5:27 pm

        Bacterial risk: Source is the plumbing code of Canada (where i’m from). My opinion does not matter in the least as all plumbing installations must follow that code. Again, check your local codes to see how to do it in your area.

        Efficiency: I am basing this on the 15 or so tankless units that i have ripped out that have heat exchangers that were completely rotted out. average age was 2 years. Your system will probably last forever because it is used very little. Our heating season lasts from November to May which really works the appliance (I’ve seen appliances not turn off for a week straight). This causes the heat exchanger to fail prematurely which then showers the control board etc. with water ruining everything (the 8-12 yr warranty only covers the exchanger itself for most brands i’ve seen not any damage it leaking causes) the customer then gets a bill for thousands of dollars, usually very similar to the amount that a proper boiler would have cost in the first place.

        Oh and your T&P valve is upside down. any sediment in the water can possibly fall onto the T&P valve over time and block it, preventing it from functioning properly (I agree it’s highly unlikely but it’s in the code book)

        How much did plumbing and Gas permits cost? If you did not pull any (here as a homeowner you are allowed to do everything on your OWN house with no issues) is there any penalty if something goes wrong (insurance not covering non-permitted installation, issues with the city when selling etc.)?

        In summary: to all readers DO IT RIGHT! look up your local codes! Something may seem simple (It may even BE simple!) but a small detail missed can be very costly.

    • Tom February 6, 2015, 11:04 am

      “combustion appliances are very inefficient when run at minimum fire when they aren’t designed to do so”

      if you take a look at newer boilers/water heaters turn-down ratios/part load efficiencies are very good. In particular, condensing boilers are provided with high turn-down modulating burners.

      running at part load only gives you a modest efficiency decrease.

      • Pete February 7, 2015, 11:18 am

        I have a system like this – it’s called floor heating where i live (eastern Europe). I get the hot water from two furnaces – one oil (for backup) and the other wood powered. It’s true that it would be bad for the wood furnace to work at minimum (because the temperature entering the system is almost never higher than 105 F, even when the outside temperature is about 10 F) but the way we solve it is we have a buffer with 800 liters of water which is heated to 180 F and the system gets the water from there. The furnace can be shut down from 22 – 07 every day, and when working it’s working at full capacity.

        • Mr. Money Mustache February 7, 2015, 12:33 pm

          800 liters of water! .. I am always jealous of the much more advanced energy-related stuff that is common in Europe. If I can add a big enough water tank like that to my own crawlspace someday, I can buffer enough heat to get through even the longest cold spell/sunless in this area without heat. Then recharge it with solar energy when it is shining again.

          • Pete February 7, 2015, 1:34 pm

            Actually my parents have a system like that at their house – 300 liter buffer connected to a furnace and two solar panels plus an integrated electrical heating element inside the buffer itself (for those cloudy weeks when furnace is not working). Solar panels are not much help in the winter, but spring till autumn the hot water for showering is free :)

          • jessica February 9, 2015, 7:16 am

            Europe is so efficient as far as living conditions go. It’s great. I wonder if we’ll ever pass them!

  • Dan Clifford Doran February 6, 2015, 10:10 am

    Excellent, I was one of those engineering nerd guys waiting for a report and this was very satisfying.

    I live only 10 miles west on the diagonal from your fair city, and also in a 50s tract ranch house. We currently use a high efficiency forced air furnace with the satanic SEER 14 AC unit to heat & cool our 1700 sq. ft. moderately-insulated abode. Our gas usage is somewhat higher than yours, but we have a 3600W 16-panel PV array on our roof for the last three years that covers 85% of our yearly electrical use (at least until AWD ‘Lectric mini-SUVs arrive) .

    Anyway, I think some of your install “suffering” could have been abated a bit with some preparation like I did w/ my 36″ crawl space:

    1) Install adequate lighting in that crawl! Running a single circuit w/ 6-10 inexpensive bare-bulb bases and inexpensive CFLs (more to keep the wattage load down than to save electricity since it will rarely be used) is a *gotta do* fora any house you plan on living in for the long-haul. That’s about a 3-hour job that will pay benefits for decades to come for any maintenance you need to do down there. While you’re wiring, put in a few outlets at strategic points to save lugging extension cords around in the future.

    2) Clean & rake your crawl dirt floor – Crawling on little sharp rocks sucks. I did this for mine, hauling out the collected rocks, 50-yr old construction nails, cig butts & other crap. Again – maybe 5 hours of work that pays over the long run.

    3) Consider laying down some white house wrap or other (cheaper?) material over all the dirt surfaces. Eliminates the dust & dirty clothes factor and makes the place brighter for any future work. Also if you do a good job sealing it against the foundation walls it *might* mitigate any radon sources a bit.

    4) I notice that your R-13 brown-paper back insulation bats don’t fill your 2/8 stud bays, just like mine. Wait for a Home Depot sale of the R-11 white-plastic *encapsulated* wall batts. These re the right witdth to fill the remainder of the bay, overlapping the joices to reduce air exhange w/ your pipes, and makes a nice smooth white ceiling for your crawl that brightens it up.

    5) Finally for bonus points, consider digging a 3′-deep walking trench to any utilities that need regular maintenance. I’m decade or so older, still in good shape, but the charms of crawling 20’ to clean a furnace filter or turn my humidifier on & off have long passed. A good used large Hammer drill w/ a spade bit will dig in a hurry. The real challenge is in removing & finding a place to dump the dirt.

    Thanks again for the fine summary.

    A fellow Colorado MMM reader.

    • Mr. Money Mustache February 6, 2015, 12:06 pm

      Thanks Dan! Among your other ideas, I love the idea of hand-digging out the crawlspace for extra construction luxury. That sounds like a truly badass task and someday I may actually do it.

      • Dan Clifford Doran February 6, 2015, 3:52 pm

        Making the crawl more usable is worthy, but before anyone gets carried away with the idea of turning a crawlspace into anything resembling a full basement using a shovel & a 5-gallon pail , beware it can be a good way to get your house declared uninhabitable or even condemned if the dig-out comes too close the foundation. This site in Denver is a company that specializes in crawl-to-basement conversions and they have some stories of DYI conversions going badly wrong:

        Be careful down there ;)

        Dan D.

  • Chris February 6, 2015, 10:48 am

    Really nice!! Have you calculated an ROI on this?? How much do you think it would cost for equivalent conventional heating on a monthly basis (not including the one time costs).

    Also, are you at all afraid of a tube bursting and ruining a section of flooring and/or a floor joist?

  • James February 6, 2015, 11:00 am

    How would this compare with some sort of electric radiant heating in the floors?

  • Steve February 6, 2015, 11:00 am

    Glad it worked. Did you consider adding an additional layer of subfloor with routed channels for the tubing? The between-joist installation you did is common, but it’s a less efficient setup with more heat loss. Not to mention a pain in the ass to install in a crawlspace.

    There are fancy radiant subfloor systems that fit together like puzzle pieces, but you can achieve a somewhat comparable result with plain plywood and a router.

    • Justin January 30, 2020, 11:53 am


      I’ve not heard of the routed flooring idea before. I have a large wooded property and am looking to install a wood gasification boiler for both radiant floor heating and hot water for showers, washing etc. I’m assuming this is applied over top of the existing subfloor and then you cover with wood, tile, etc.

  • Frugal Buckeye February 6, 2015, 11:00 am

    This is great. I’ve been anxiously awaiting the results of this experiment. We have radiator heat in our current house and it is certainly great when it works, but it is an older system and some of the loops have issues which can be a pain.
    Because of it’s benefits I’ve thought about trying something similar and placing more in floor radiant runs.

    Being in Ohio we have a bit more humidity than you do in Colorado and with a radiant heat system you need a separate source for AC. Do you have any Mustache approved ideas? Other than sucking it up and living without AC for just a few months in the summer ;)

  • cynthia February 6, 2015, 11:24 am

    Hi MMM! I know you love to DIY things and that engineers love to reinvent the wheel.

    But to make life easier, and maybe pump up those curious grey cells, I’d recommend checking out the heating options we use here in good ‘ole Europe–because no one here uses forced air furnaces! There are tons of options–your radiant floor option here is ONLY recommended in places with longish/serious winters, because in southern places which can have mild sunny days alternated with cold ones–well, the radiant floor system doesn’t change temp very fast, so my brother in law on the Mediterranean who was very silly and didn’t listen to local experts and put in this system, has to open his windows in the winter on a less cold day because his radiant system can’t deal with rapid temp change…
    Radiant floor systems are GREAT AND EASY when in new construction: here they just pour the concrete slab, wind the piping over it once dry, then fill in the spaces with some fluffy insulating stuff, pour concrete around, then lay on the tile or wood floor or whatever.
    We have one Biomass central heater for our entire neighborhood, an new Eco-village concept thing, which burns pellets of reclaimed sawdust at high efficiency, then complements with solar panels that use mirrors to concentrate. This central heater heats to 70 deg celsius and arrives at our house at that temp for the radiators and hot water showers. BUT because the house is well-designed (not just well insulated but with bio-climatic principles i.e. shade tress on the south side, windows facing south and not north, you know, how they knew to build in the old days! THERE IS SNOW ON THE GROUND, IT IS FREEZING ZERO CELSIUS OUTSIDE, AND I STILL HAVEN’T TURNED ON THE HEATING! Isn’t that great!?? and my house is a lovely 68 deg F inside…
    They also use here geothermal, something they call Puits Canadiens, these are pipes that go down to the earth’s constant temp and then back up to your house…anyway, there’s tons of different ideas floating around over here, partially to comply with more stringent environmental regulations…
    thought that might pique your curiosity! :) or perhaps you know this already…

    • Emily zefrencharchitecte August 14, 2015, 3:48 pm

      I simply must correct that Puits Canadiens refers to long, burried (low-depth) air ducts that refresh or pre-heat (air) coming into the building. The venitlation system needs to be double-flux (forced-air). Most French ventilation systems are extraction-only. Oh and in Canada they call it Puits Provencaux.
      It is not geothermal energy in the sense that it does not use the energy of the (core of the) Earth, but rather geosolar energy, the energy absorbed seasonally by the surface of the earth.
      You live in a good sensible eco-neighbourghood, enjoy !
      There are also energy-wasting machines ( older buildings) in Europe. Lots of work to be done for us good-willing people, both sides of The Pond. :)

  • Chris February 6, 2015, 11:29 am

    My 4,500 office building which I bought in 2012 is heated by radiant in-floor heating similar to your setup. Two BAXI tankless water heaters (also known as on demand boilers), four thermostats, two floors (both have the in-floor heating. Backup is two rooftop heat pumps that also provide A/C in the summer.

    We have loved this system the last two winters. Floors are cosy and I take my shoes off as soon as I come in every day. The heat pumps are set to run for 10-15 minutes every weekday morning just to bring the temp up a few degrees on super cold mornings. Then all day there is no dusty air blowing around and the whole office is very quiet and warm.

    Being a commercial building we pay ridiculous rates for natural gas and electricity but I know the radiant heating is saving us a ton while also being really effective. The only downside so far is that on very cold nights the boilers fire like crazy to heat the floors since the thermostats are all set to work off air temp at 5′ up off the floor. Then this heat ever so slowly heats up the surrounding concrete and continues to heat the rooms all day. On sunny days the south facing room that get a lot of solar gain via the floor to ceiling windows and end up too hot and we ended up installing Hunter Douglas cellular shades on all those windows (and its a lot of glass). Decent solution but it wasn’t cheap.

  • Sam February 6, 2015, 11:36 am

    My biggest concern would be the water smelling/tasting bad. Not being able to use any warm water in cooking or even just drinking (when you have a sore throat perhaps?) would to me seem like enough of a reason to not do it.

    Do you think the taste is coming from the PEX or from some of the radiant heating components?

    • Justin February 6, 2015, 1:50 pm

      For what it’s worth, you’re generally not supposed to use hot water for cooking anyway, as the hot water will tend to have more minerals dissolved in it, and affect the taste of the food. You’re supposed to start with cold water and bring it up to temperature every time.

      As far as drinking goes… I tend to heat up a mug of water in the microwave whenever I make tea (again, starting with cold water), I don’t know when I would last have drank hot water directly from the faucet.

      • dand February 6, 2015, 3:37 pm

        The advice for not using hot water for drinking & cooking is only really applicable to tank water heaters. There’s often a considerable pile of mineral sludge that builds up in the bottom over the years. Since the hot water sits on top of it for hours or days, any dissoluble content will get picked up by the hot water & get delivered to your faucet.

        Demand heaters (aka ‘tankless’ heaters) have no reservoir for sludge accumulation. The hot water is delivered to the faucet as quickly as the cold water and from the same source pipe. They are equally safe for drinking & cooking, just different temps.

      • woodnclay February 6, 2015, 4:13 pm

        I am wondering what adds minerals to water at the heating stage? Boiling water concentrates existing minerals due to loss of some water to steam, until you end up with calcium carbonate (or whatever is dissolved). If hot water is stored in a copper tank then there may be copper present in the water (or lead?).

        Hot water in MMMs pipes may “leach” out more stuff like MBTE from the PEX pipe – hence the smell.

        Also, there’s the possible presence of chlorine (or other residual) in the drinking water feed which may react with stuff from the PEX.

        So, as a former water treatment engineer, I wouldn’t drink the hot water from the PEX pipes or a copper cylinder. But I do consume (in cooking) hot water from my own hot taps (faucets) which travels in copper pipes, heated on demand by a dual circuit boiler.

        • Robert February 7, 2015, 10:44 am

          Woodnclay, you make some good points. I think in general that with enough water usage/flow, the amount of contaminants leached out into a given volume of hot water will probably be low, without health effects. However, if you had the water standing in the system (hot) for awhile, especially when new, and then used it immediately for cooking, I assume the concentrations would be quite a bit higher. There may not be health effects, but off-taste and odor could be significant. I have a bit of experience with polyethylene technology, so let me elaborate a bit on this topic…

          We are all familiar with the taste of water in the plastic milk-jug type containers, which are made of polyethylene (PE) like PEX is. It can taste pretty nasty compared to water from polyester terephthalate (PET) or polycarbonate (PC) water bottles, especially if they have been stored in a hot warehouse or truck before you bought them. The off-taste and odor are associated with small molecules of various organic compounds, typically degradation products of the PE due to oxidation during manufacture of the bottle itself. PE is basically just a high molecular weight analog to paraffin candle wax, so if you have ever snuffed a paraffin candle and smelled the resulting smoke, that is pretty much the smell trapped in a PE bottle after it is made by hot extrusion and blowmolding. The off-taste and odor are unpleasant, but may or may not be associated with health effects.

          Back to PEX… PEX is actually not one material. It is a generic name used by the pipe industry for crosslinked polyethylene (PE). But there are thousands of different grades of PE made by various processes, using various catalysts and monomer compositions. PEX pipe is typically from high density polyethylene (HDPE), which just means it has a high crystallinity. That in turn results from the fact that these PEs are made with a catalyst that avoids formation of long-chain branching (as occurs in low density polyethylene, or LDPE), and also because they are made with little or no comonomer (i.e., they are made of pure ethylene monomer, not including propylene or butene or other comonomers sometimes used to modify PE properties). All of these variables have an effect when the polyethylene is subsequently processed into parts, since different polymer compositions alter the polymer’s response to the process itself, and change the type of degradation products that are formed. Furthermore, the crystallinity of the product changes the rate at which these degradation products diffuse (and leach) out of the product; as you might imagine, higher crystallinity leads to slower diffusion rates (but also makes that pipe stiffer and harder to uncoil and pull through your joists, MMM!).

          Once you have the PE, along with whatever small molecule degradation products formed during the high temperature pipe extrusion process, you have to crosslink it. There are 3 different processes used: PEX-a uses peroxide crosslinking; PEX-b uses silane crosslinking, and PEX-c uses electron beam radiation crosslinking. (Amusingly, an industry document describing the PEX-c process refers to “a dose of high energy electrons”; did they think that sounded less scary than “radiation”?!). Each of these processes leaves different residual products in the PE.

          Peroxide crosslinking is accomplished by mixing a small amount of an organic peroxide with the PE and heating it until the peroxide decomposes, producing free radical products that crosslink the PE. There are many different peroxides available, and each has its own decomposition products. A common peroxide for crosslinked wire and cable insulation, for example, is dicumyl peroxide, which leaves residues like cumyl alcohol and acetophenone. Acetophenone has a strong odor associated with it. (Sniff a brand new pair of Crocs sandals to know what I mean). Other organoperoxides produce other products. For example, t-butyl peroxide leads to t-butanol and methyl t-butyl ether (MTBE), which has been associated with a gasoline odor that consumers complained about in PEX piped water (http://tinyurl.com/njcwde5). Besides effects on taste and odor, some such decomposition products have health effects, and their use in food contact applications is thus regulated.

          Silane crosslinking refers to a process whereby an organosilane is grafted onto a PE polymer, then in a second step, exposed to moisture (typically in the presence of an organotin catalyst) which leads to crosslinking via a hydrolysis/condensation reaction. Vinyltrimethyoxysilane (VTMOS) and vinyltriethoxysilane (VTEOS) are two of the more commonly used silanes. Because the silane first has to be grafted to the PE via a free radical process, organoperoxides are typically used for that. Thus, PEX-b can be expected to have both peroxide decomposition products (as PEX-a has) and silane decomposition products. Examples of the latter include the VTMOS hydrolysis product methyl t-butyl ether (MTBE) and the VTEOS product ethyl t-butyl ether (ETBE). You may recognize MTBE because it has received a lot of adverse publicity in light of its role in groundwater contamination resulting from gasoline spills (it is used as a fuel additive). Trace amounts of tin from organotin catalyst could also be present. PEX-b is the most common type according to http://www.pexuniverse.com/content/types-of-pex-tubing.

          Electron beam radiation crosslinking involves no added chemicals. The high energy radiation creates free radicals in the PE itself which then recombine to form crosslinks. However, the high energy process can still lead to degradation and formation of oxidation products as these free radicals may also react with dissolved oxygen in the polymer to form various decomposition products not unlike those formed during hot extrusion (i.e., back to the milk-jug odor/taste).

          This very brief introduction is intended only to highlight the complexity and variety of the processes involved in making PEX. Thus, perhaps it is no surprise that there is a wide variation in the taste and odor of water in PEX made by various processes and manufacturers. This is further complicated if the manufacturers or installers use plasticizers/lubricants and other chemicals to assist in processing, cleaning, etc. If the pipe is exposed to chemicals in the ground (if in ground contact) or spills, these can be absorbed by the plastic and later leached into water passing through the pipe. And, the water itself may contain chlorine or other chemicals that can interact with the various PEX byproducts listed above, resulting in a further profusion of small molecules that could potentially increase off-taste and odor or potentially toxicity.

          On the regulatory side, NSF regulates pipe. There are many tests associated with the longevity and physical integrity of the pipe itself (nobody wants pipes cracking after 10 years and leaking water). There are also tests for specific contaminants. NSF-61 governs drinking water health effects. Any PEX pipe used for potable water should have this certification. (There are PEX pipes sold for hydronic heating applications that are not intended for use with potable water, and shouldn’t be used in an open loop installation as MMM described). NSF-61 includes a hot water static exposure test with subsequent analysis for regulated chemicals at above threshold concentrations (see reference below). I should think this would be sufficient evidence for safety, but it should also be kept in mind that some individuals may have enhanced sensitivity to specific compounds (i.e., people with MCS), and also that as science advances, we sometimes learn that compounds once thought safe no longer are considered as innocuous (e.g., BPA).

          For an application like MMM’s, you’d want to be sure your PEX was certified for hot potable water use. I personally wouldn’t be overly concerned about toxicity if using PEX with NSF-61 certification. However, it makes sense to flush the pipes if standing for a long time, as a precaution. The certification testing can’t test all the possible contaminants, and the health effects are not fully understood in all cases. I wouldn’t like off-taste and odor in my water in any event, and might choose to install a filter for that reason. (It is also worth remembering that one shouldn’t look at PEX in a vacuum; copper pipe can have lead issues, PVC has additives too, etc. We live in a chemical environment and interact with chemicals everyday. We can’t eliminate them; just try to be smart. What you eat and drink is likely far more important than trace contaminants. i.e., there is more ethanol in a beer than in your PEX piped water).

          It would make sense to buy a certified PEX that also had low off-taste/odor associated with it, but where can you find THAT information?! That’s a problem that perhaps a consumer organization can address in the future. Here’s an article about a study being conducted at the Univ. of S. Alabama to compare several PEX pipes for taste and odor: http://tinyurl.com/q4j44zb.

          Something I haven’t read about is what effect methane and other natural gas components have on the longevity of PEX pipe or the leaching of PEX byproducts. When you see videos of flames shooting out of faucets allegedly due to hydrofracking-related contamination of aquifers, it does make me think that in certain regions of the country this should at least be considered. A low molecular weight hydrocarbon in the water could act as a solvent in PEX pipe, accelerating leaching processes and also potentially accelerating the aging of the pipe itself. (Solvents typically accelerate stress cracking in plastics; they may also leach out antioxidants/stabilizers).

          Here are some other references of interest:
          California often leads the nation on health/environmental/consumer advocacy. Here is a summary of some PEX related issues: http://calpipes.org/ProtectingCalifornians_PEX.asp

          A Norwegian study compared PEX from all 3 processes and concluded that they were safe for potable water use, even if some had taste and odor issues. Interestingly, during initial use, some had MTBE levels above the upper limit for potable water advised by the US EPA, and this persisted for several months. (I might add that any such study looks at only a small sample of the products on the market, in this case, the Norwegian market. Results may vary in other products. Nevertheless, this study is reassuring).

          A green builder’s perspective, which seems pretty fairly balanced to me: http://tinyurl.com/79bv3dq

          A summary of the various tests/certification standards for PEX, including NSF-61: https://plasticpipe.org/pdf/tn-17_crosslinked_polyethylene_pex.pdf

          Details on the NSF-61 testing and what markings to look for on the pipe to ensure compliance are found here: http://plasticpipe.org/pdf/pex_frequently_asked_quesions.pdf

          • Sam February 7, 2015, 2:44 pm

            What an incredibly thorough write-up! I sincerely appreciate it, it definitely shows some of the downfalls of a full-pex system.

          • Kenoryn February 9, 2015, 12:48 pm

            Wow, thanks for this summary! I learned a lot.

            Speaking of silanes: In university I did my thesis on the prospects of sol-gel technology for removing contaminants from water. I was looking at metals, but I imagine it could be used for organic compounds as well. We embedded nanoparticles in the sol-gel matrix to try to target the specific metals we were looking to remove (arsenic and selenium) but found the sol-gel matrix removed them itself and the nanoparticles had very little effect. But the principle of embedding something to adsorb certain compounds still makes sense. It can be applied as a coating, so maybe someday it will find use in plumbing systems to not only prevent contamination from plumbing components, but remove other by-products of water treatment as well so your water comes out of your tap cleaner than it came into your house. Of course you’d have to make sure it didn’t remove minerals as well, or that could be a problem, although great for people who get kidney stones.

          • Chuck November 25, 2015, 6:15 pm

            I guess I am out of luck with my open system. I put E verhot O2 Pex in my small slab in the basement, and I don’t think it has the NSF-61 rating. I was going to eventually hook up my Polaris high efficiency hot water heater and do water heating and radiant. Rats.

  • Jameson February 6, 2015, 11:38 am

    I rebuilt a bathroom about 10 years ago with an electric radiant heat under slate tiles. This is in the mountains of New England.

    OMG WHAT AN AMAZING EXPERIENCE. There is nothing like walking into a bathroom in the morning to a warm floor. I didn’t have any need for bath mats because the floor itself was warm. Things dried INSTANTLY, I never had mildew problems or anything. If I ever re-do a bathroom, I’d do it again.

    The company I bought from took the dimensions of my bathroom and gave me a custom shaped roll down electric “blanket” that I put between the subfloor and the tiles, basically putting in between a layer of quickset. A little controller sat on the wall and I could program a daily schedule of “off,” “warm,” and “hot.” It saved a ton of room in the bathroom to be without baseboard heaters, and it really was the best thing I’ve ever done to a bathroom.

  • DIYorDie February 6, 2015, 11:47 am

    You mention that you have problems with hot water being fussy with a system temperature of 140 degrees. We also know that you use very little hot water in your house. It might be worth it to install a mixing valve between the boiler and the domestic hot water take off.

    I did this with my system as a safety, I have small kids and I don’t want scalds. Our Baseboard radiant system has to be at about 160 to heat the house. This allows me a wide range of adjustment on the domestic hot water.

    Also, when my wasteful brother-in-law comes to visit I conveniently turn the water temperature down to 90 after 3 minutes or he would sit in the shower for an hour.

    This is what I used: http://www.supplyhouse.com/Webstone-75103W-3-4-Sweat-Thermostatic-Mixing-Valve-Lead-Free

    • Dave February 8, 2015, 6:01 pm

      Was going to suggest this as well…..I have a hydronic forced-air system using an open-loop, and a mixing valve is used for the domestic to solve this problem. AKA a “thermostatic mixing value”.

  • DoItYourself February 6, 2015, 11:48 am

    Congrats on the success of your radiant heating system. I am intrigued by the effectiveness of your passive solar heating. I would love to see a similar engineering analysis post on the passive solar heating. Solar radiation received for a given day of the year, solar radiation captured, area of the windows etc. It appears as though the solar heating meets your needs during much of November and February. What percentage of your heating is due to the solar during January and February?

    • Joe Average February 9, 2015, 3:28 pm

      Had an opportunity to “bench test” a evacuated tube solar water heater system and was surprised to see steam rising from a leaky connection I caused very quickly. Had the thought that a system like this plumbed into a heated floor or radiators might really heat a home well…

  • Texas Jim February 6, 2015, 11:57 am

    I am curious why you didn’t choose something with better thermal conduction than plastic? Given the transfer issues that you alluded to, is this substantially different from the classical forced water systems with standing radiators like the one with which I grew up in the 1950s in NJ? That home had few energy efficient modifications other than storm windows and was really toasty. If I understand, you have ~1,500 sq feet with 1,000 currently working in the system..

    In Texas the energy problems are primarily for cooling. We have 2,300 sq feet which is all electric. With retirement approaching we invested heavily in a remodel to make the place liveable and enjoyable. However, along the way we made as many efficency adjustments as possible. The result has been surprising. In a part of the world where neighbors have $600/month electric bills ours averages about $110/month. Our biggest usage again surprising for TX is for heating in winter. I would be interested in any other ideas, particularly to address the winter. We raised the ceilings throughtout the house to 11 feet and added 9 casement dormer 4E windows in the roof. We replaced all of the remainig 30 year old single panes windows with similar casements including 15 new windows where there were none before. While adding the dormers, we replaced the roof decking with foil backed panels, added soffit and ridge vents. In summer cool air passively enters under the eaves, is conducted via baffels along the underside of the roof and out through the ridge vent continuously flushing the attic space (which is now quite small). In the evening we can open the upper and lower windows and passively hot air escapes up top and is replaced by the cooler evening air from below. We added a full length porch ( for siping merlot) to the south side of the house to reduce heat gain in the summer but permit lots of light from the winter sun. We also moved the ait conditioner (heat pump) to the shady (east) side of the house. The cost of the total remodel was well beyond any possible payback or breakeven but we are very happy with the results having turned a dark depressing wood paneled rancher into an open, airy, passively lighted dream home. PS: we have replaced an ancient water heater with a well insulated high efficiency model and have transitioned all lighting to LEDs. Any other suggestions would be appreciated.

    • DIYorDie February 6, 2015, 12:07 pm

      You really can’t do much about your heat in the winter. Those high ceilings let all your warm air rise. The only option to maybe make things more comfortable would be to put a number or ceiling fans to blow the heat back down.

      • Texas Jim February 6, 2015, 3:58 pm

        DIY…thanks for the input. We do have fans in most of the rooms and reverse them in winter. It doesn’t help as much as we would like but perhaps we need to speed them up a little.


    • Debtless in Texas February 6, 2015, 1:07 pm

      Thankfully, this Texas winter has been fairly mild (at least in central Texas)!

      We installed ceiling fans in every room to circulate some air to keep from having to use the AC/heater too much. Having such tall ceilings is bad news for rising hot air but sounds like a dream in the summer.

    • HenryDavid February 7, 2015, 9:59 am

      if you look at houses in hot countries they all maximize passive cooling. I only know southern Europe. There they use high ceilings, and very clever adjustable window shutters that block sunlight but allow airflow. Also ceiling fans. And porches, or just sitting on the front steps, sipping merlot or chianti or whatever until 1 am.
      You open up the house at night, close it up on summer days. Most of the cooling gets done that way, but you see small heat exchangers and single room AC gets used a bit too. No one can afford to run full-on AC at European electricity rates. North Americans can learn a lot from the old world.

    • Allise August 3, 2015, 1:06 am

      How do you open and close your upper windows? I had designed our new home with upper windows to take advantage of stack-effect cooling, but motors for the out-of-reach windows seem cost-prohibitive.

      • Emily zefrencharchitecte August 14, 2015, 4:04 pm

        A perch would do. (did I get the word wrong ? a long stick )
        One that stays attached to the window or a retractable one that you can hide away.
        or a rope similar to horizontal blinds. They ere fairly common (not standard but common) in Europe.
        Look into importing european windows if need be. But if you have the choice to have motorized windows, the same model with a perch/handle/crank probably exists.

  • WageSlave February 6, 2015, 12:51 pm

    MMM, do you happen to know what the *whole wall* R-value of your house is (i.e. not just R-value of cavity insulation, but taking into account the studs’ thermal bridging)? And windows, floor, and roof for that matter?

    Just curious, as what I’ve read is that the utility of radiant-heat systems like yours decreases as the thermal performance (insulating plus sealing) of your structure goes up. In other words, a super-insulated, airtight house will rarely need any mechanical heat. You’ve already observed this for your typical winter days. Even in colder climates than yours, I’ve seen people talking about adequately heating a high-performance 2000 ft^2 house with only a single air-source heat pump (a “mini-split”). (Disclaimer: this is all based on hearsay, no practical experience.)

    I maintain a list of “dream house features” that I intend to implement in a new house (in the future when I reach FI). When I first read about radiant heat (here), I immediately added it to my list. But the top item on my list is ultra-high performance (PassiveHouse, R-2000, LEED, etc depending on budget), which might make for very few opportunities to use a radiant heat system like yours.

    • Emily zefrencharchitecte August 14, 2015, 4:20 pm

      You are perfectly right that sophisticated heating systems are not needed if the house if perfectly insulated. MMM’s house is a renovation and he can improve it (and chose it well for south-facing window opportunities) but hardly make it passivhauss-standard without losing its “cachet”.

      In YOUR dreamhouse, you won’t need much heating if any. To avoid cold floors, put “warm” materials down (linoleum, wood, cork, bamboo, of course carpet -wall to wall or rugs- etc) especially on the ground floor. A real basement or a slab are more comfortable (for your feet, not for maintenance or cost) than crawlspace.
      In passive houses, a small woodstove or insert would suffice if the house is designed for good natural air circulation. I have found that friends light fires more often than necessairy and thus need to open windows (that’s good to keep indoor pollution low, so why not) in the coldest days of winter.
      also, if you have a staircase up to (bedrooms usually), have a way to close it off. It gets toasty and even too stuffy to sleep.
      Those same (not even passive standard on paper) friends (3 couples/families now) tell me cooking and the water heater & pipes give off are more than enough. Some are planning to get more energy-efficient appliances because the old parent-inherited ones are giving off too much heat.
      Wonderful daydreaming material for us who wear slippers and sweaters, right ?

  • Parker February 6, 2015, 12:53 pm

    My issue with DIY type systems like the one you built is that if I was to ever buy your house, I’d rather have some type of industry-standard and licensed contractor installed system than one that someone with little HVAC/plumbing experience created. Not that it couldn’t be the world’s best heating system, but by being ‘custom’ it is best understood by the person who created it. That to me is the problem with many people’s ‘off the grid’ projects.

    • Mr. Money Mustache November 15, 2015, 4:53 pm

      You have a good point Parker – sketchy DIY work will often be an issue for future buyers.

      In this case, I did follow standard procedure and the work can easily be inspected for functionality and safety. Hot water tubing is a perfectly normal way to heat a house and it is gaining in popularity.

      As for the licensed contractor issue: it doesn’t matter who does the work, it can be well or poorly done either way. The key is in inspecting the work (and checking for approved permits and inspections as applicable).

      In my case I AM a licensed general contractor so it should be even less of an issue, but it is totally normal and appropriate for skilled homowners to do their own work, just as it is appropriate to cook your own food or work on your own car instead of hiring a mechanic.

  • Tawcan February 6, 2015, 1:12 pm

    It’s great to hear that the radiant heat system is working out so well. I can see how warm feet will make you feel warmer even though the room temperature is not at 20C. A friend of mine is building their family house and using radiant heat system throughout the house which I thought was an excellent idea. My in-law’s place has radiant heat system as well and I really like it. It would take a lot of money to change our heating system to radiant heat system I think. Having said that we already have a tankless system for hot water so we already have part of the set up.

    I didn’t realize how efficient the radiant heat system is. Very cool to see a real world usage example. Thanks for sharing.

  • Kayla February 6, 2015, 1:20 pm

    I’ve heard of this idea before, but I like how you told us exact steps and details. This was a really great explanation. Thanks for sharing!

  • Swepri February 6, 2015, 2:50 pm

    A small input from a cold Sweden. We built an extension to our house a few years ago and put the floor heating tubes in the concrete floor. Three zones, each controlled by its own thermostat. As the heat source we use a heat pump with a 120 meter drilled hole into the mountain. This is pretty much the standard way to build nowadays in Sweden. Very comfortable and consistent heating.

    We also put an oversized radiator with fan on the bedroom wall to provide cheap air conditioning in the summer. The fluid (2-6° C) is simply circulated directly from the drilled hole up to the convector in the bedroom.

    A few pictures: http://paj.smugmug.com/Architecture/Tillbyggnad-1110

  • Chachatat February 6, 2015, 3:59 pm

    I have been designing a new house for the last year. I have been planning to do hydronic radiant floor heat since day one of planning. I live in the Oregon Valley where the climate is pretty temperate although we did get temps below 0 deg last year and have definitely hit above 100 deg during the summer. My current plan is to install a gas combi boiler (Navien NCB 240 to be exact) to run separate hydronic heat and have separate DHW. Does anyone have a story about combi boilers? I am mostly curious as to how well the tankless system or a combi boiler handles total household flow. Have there been any issues with hot water for a bath as well as maintaining the floor or sinks? Also, I was told by a family friend to have one circulation pump for downstairs floor heat and separate one for upstairs. Is the one pump handling both your upstairs and downstairs and managing a-ok? My hydronic will be installed under the sub floor and the house will have hardwoods (no concrete slab for me) and the siding insulation is going to be above R30, so heat loss will be pretty minimal. I have also heard a lot of arguments for/against putting aluminum plates or insulation in under the pex. The consensus I got was that heat rises and there is little concern of heating the space below the pex and what heat loss there may be does not warrant the cost of the insulation or aluminum. Any arguments on that front?

    • Anom February 9, 2015, 9:46 am

      I have the same question regarding putting in insulation under pex. I have a similar pex in-floor heating system in the ceiling of my basement with aluminum plates installed, however I’m undecided as to whether insulation under it is really necessary as heat should rise regardless.

    • Emily zefrencharchitecte August 14, 2015, 4:39 pm

      There are three ways heat is exchanged (at our level, like in a house not it atoms ok?)
      radiation, convection and conductivity. the opposites, in order would be reflection, batting insulation or cutting the flow of air, and touch insulators (gloves, shoes…)

      Radiant heat radiates in all directions (much as a source of light, indeeds it radiates infrared light…) only less-than-half will radiate to your floor, the other half will heat up the dirt underneath in your crawlspace.

      “heat rises” is short for “heated air rises”. Like in convector radiators (baseboard of ventilated or other).

      In your crawlspace, I will hope ventilated for sanitairy reasons, the hot air will be lost.
      You absolutely must insulate the piping and thus creating a heat pocket that warms your floor. Your floor then radiates the heat up to you and also has a nice surface temperature (conduction of heat).
      The aluminium reflects the heat back up (in the case of a floor) serving the same purpose but not insulating the heat in. Batting if cheap and effective between and over joists.

      If you are planning on putting it up on a second story floor, then you can skip the insulation. it will radiate both up and down.
      if your floor is on a crawlspace and uninsulated you should tackle that right after you insulate your roof!!!

  • Dave February 6, 2015, 6:14 pm

    You should try adding Honeywell mixing valve to address the shower temp issue. They automatically mix cold water with the hot to achieve a set temperature for the domestic hot water feed, eliminating the scalding risk. Plumb it in after the dishwasher line if you can, then the dishwasher will still get the extra hot water it likes. Look at a Honeywell AM101. They work great!

    • Mr. Money Mustache February 7, 2015, 8:04 am

      Thanks Dave, I like that idea! Definitely a good use of $100 and a few minutes.

  • LeisureFreak Tommy February 6, 2015, 7:39 pm

    Pretty cool setup. Where my family is from in Midway UT there was a lot geothermal setups like this but the hot spring water was the heat source. My sorry 28 year old house here in CO is fully finished so doing anything other than getting a high efficiency forced air furnace and efficient gas fireplace inserts would most likely be cost prohibitive. My first house that I owned was outside of Slat Lake City UT and I finished the basement to include an EPA compliant (catalytic converter) wood burning stove and ceiling vents with thermostat controlled fans that would pull air upstairs once basement room temps hit 73. We heated the whole house burning Mill Oak ends from the cabinet shop that I got for $40 a cord. Paid year long the $30 minimum natural gas bill and only paid out $120 (3 cords) for the wood we burned. I would load the stove up while we were home and awake and nights or when away just let it default to the forced air furnace when the stove burned out.

  • dave February 6, 2015, 10:11 pm

    In many rural cities wood smoke in the winter is a huge health problem. And the particles you breath in are so small they get deep in the lungs. This has to stop


    • peachfuzzstacher February 12, 2015, 6:41 am

      In many rural and even suburban cities, only expensive and non-renewable fuels like oil and propane are available. If you use an EPA rated wood stove instead of an open fireplace or old stove, the efficiency is much higher and there is minimal smoke due to higher burn temperatures. It sounds like you’re judging all wood burning equal.


  • gallygirl February 7, 2015, 5:52 am

    Fascinating stuff (though I didn’t follow all of it).

    However I couldn’t quite get over my disappointment in discovering I’d misread the beginning of the post and it wasn’t a home-heated brewing system :(

  • Mike Mitchell February 7, 2015, 6:54 am

    With this type of system it is much more efficient to keep this system running consistently. You lose a large amount of the efficiency hydronic radiant heat provides when you are trying to heat the space from scratch. Since the system is designed to provide heat from the radiation of the objects in the room rather than through convection in the air, a consistent temp must be maintained to reduce your delta T. That is why supplementary heating is usually provided along side these systems (radiant panels) so that the odd temperature periods(spring and fall) it fills the gap for the system when it is cool and night but hot during the day.

  • Scott February 7, 2015, 7:08 am

    Dear MMM, i have recently found and read all your articles. I would love to see your bad ass breakdown of a DYI PV solar system. From what I have been reading your location is ideal for a ripe return on the investment. A rencent report suggests that an investment in PV could give an equal ROI as investing in an S&P 500 index find.

    • Michael February 12, 2015, 4:09 pm

      In Iowa DIY PV Solar system with Federal 30% and State Rebate 18% makes ROI of about 10-12%.
      For one year Alliant Energy Rebate also had a rebate of 50% of the remainder –and I still didn’t pull the trigger, arrg.

      For instance on civicsolar.com you can price out a 8kw system at approximately $13,000 with “dealer” rates (just ask for it) including panels, roof racking, clips, micro inverters, enphase wiring, junction boxes etc. Then tack on $1,000 for some major wiring to your main panel/inspection, and its at about 14k.
      30%+18%=48% savings
      so 52% of 14k =$7,280
      A 1kw system in Iowa averages 1,350 output per year.
      1350 x 8 =10800 per year or about 900Kw/month
      900KW x .13 (electric rate) = $117 per month savings
      (not accounting for any system failures or electric rate increases)


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